Scanners typically are used to scan a document to create a scanned image and associated image data. The scanned image may be printed, faxed, displayed on a computer monitor, used by a computer program or the like.
Scanner speed performance can be limited by the amount of light required to illuminate the document. In particular, as the scanner speed increases, the light exposure time decreases proportionally. Therefore, a faster scanner typically requires increased illumination intensity across the entire document to be scanned.
Due to the inherent optical properties of typical scanner lenses, prior art scanners attenuate more light at the edges of the scanning field than at the center of the scanning field, thereby creating the known vignette effect, wherein the edges of the scanning field only receive about 40 to about 60 percent of the light received in the center of the scanning field. A graphical illustration of illumination intensity versus field angle in a prior art scanner subject to the vignette effect is shown in FIG. 1.
One prior art solution to address the vignette effect is to attenuate the center of the scanning field by about 20 to 40 percent. The center attenuation is achieved using an absorption band on the lamp reflector(s) and/or a shading plate near the lens. A graphical illustration of illumination intensity versus field angle using center attenuation is shown in FIG. 2. While a somewhat flatter illumination profile is achieved, the center attenuation design approach may waste light and, therefore, may be inefficient.
Accordingly, there is a need for a scanner and associated system for achieving more uniform illumination intensity versus field angle without the potential inefficiencies associated with center attenuation.